Modulator for radiotelephones



March 930. H. H. c. DUNWOODY 1,750,579

MODULATOR FOR -RADIOTELEPHONES Filed April 20, 1926 l i l l l l l l l ll P INVENTOR WITNESSES Z EH61 Dunn 00%,

J" ATTORNEYS Patented Mar. 11, 1930 PATENT OFFICE HENRY H. C. DUNWOODY,F INTERLAKEN, NEW YORK MODULATOR FOR RADIOTELEPHONES Application filedApril 20, 1926. Serial No. 103,397.

This invention relates to improvements in radio telephony, and one ofthe objects thereof is to provide an improved modulator for use inconnection with the telephone transmitter, use being made of theproperty of certain substances to vary in electrical conductivity underthe action of heat or light in producing a changing current in thetalking circuit.

Other objects and advantages appear in the following specification,reference being had to the accompanying drawings, in which Figure 1 is adiagram of an oscillator circuit illustrating the use of the improvedmod- 35 ulator.

Figure 2 is a detail sectional view of the improved modulator, theassociated talking circuit being indicated diagrammatically.

Figure 3 is a diagrammatic and sectional view of a slight modification.

The principle of radio telephony, telegraphy and the like consistsfundamentally of changing the amplitude of the high frequencytransmitting or antenna current.

The frequency of the diaphragm (using the telephone for illustrationherein) is impressed upon the foregoing antenna current,

and inasmuch as it produces said change in amplitude is known as themodulating frequency. v

WVithout encumbering the present description with unnecessary details,it is suflicient to say that practice has demonstrated thenonfeasibility of impressing the transmitter diaphragm frequencydirectly upon the antenna current or carrier frequency, because the latter will be changed but little so that both the quality and distance oftransmission will be poor. Again, it is deemed suflicient to say thatpractice has demonstrated the need for a modulating device by which themaximum effect of the diaphragm can be imposed upon the antenna.

As is known, modulating devices exist in a number of shapes. Anoscillating vacuum tube is commonly employed in the required capacity.Magnetic modulators are employed to good advantage also. As previouslystated, the herein improved modulator employs the peculiar property ofvariable conductivity under the action of light or heat of certainsubstances, for example carborundum, selenium, magnetite, etc. Figure 1illustrates an oscillator circuit of more or less conventional form. Ahigh frequency current is impressed upon the antenna 1 by the powersource 2 which in this case is an oscillating vacuum tube. The circuitof the grid 3 includes the secondary winding 4 of a transformer. Thetransformer has a primary coil or winding 5.

For convenience of description, the circuit 6 is known as the talkingcircuit, and it is in this circuit that the improvement exists. Thecircuit includes the telephone transmitter 7 which is regarded as havingthe well known arrangement of vibratory diaphragm and compressiblecarbon granules, the function of which is to vary the current in thetalking circuit originating from the battery 8. The circuit includes afilament 9 and a rheostat 10. A given setting of the rheostat willpermit a given. current flow in the talking circuit, supposing thetelephone diaphragm to be at rest, but vibration of the diaphragm willincrease or decrease the current flow above the initially given value sothat the heat or light of the filament 9 will vary to correspond.

Interposed between the foregoing talking circuit 6 and the primary coil5 is a pair of tubes 11 and 12 in concentric relationship (Fig. 2). Thelatter is larger than the former, permitting the intervening space to befilled with a substance 13 such as carborundum, selenium, magnetite,etc. which varies in electrical conductivity under the application ofvarying heat. The substance is subjected to the heat of the filament 9which is inserted in the tube 11 but in spaced relationship thereto. Thefilament maytake any desired form. The substance 13 is held in placebetween the tubes by any suitable end closure.

Suitable binding posts 14 and 15 serve as connectors for the primarycoil 5. A battery 16 and rheostat 17 are connected in this circuitwhich, although it contains the primary coil 5 of the transformer issecondary in rela tionship to the talking circuit 6.

The battery 16 acts as a booster t0 the secondary talking circuit. Therheostat 17 will permit any desired current flow through the coil 5 andsubstance 13. But inasmuch as this substance is still subject to theheating effect of the filament 9 it follows that varia tions in the heatwill vary the conductivity of the substance so that the current of thebattery 16 may flow with more or less facility. It is thus apparent thatthe activity in the secondary talking circuit is dependent upon theactivity in the primary portion 6 of the talking circuit.

In Figure 3 the substance which varies in electrical conductivity underthe action of heat or light now takes the form of a tube 18. Thefilament 9, which may be regarded as the same (Figs. 1 and 2) isinserted in this tube. The second concentric tube 12 and the filling 13are dispensed with, the secondary talking circuit, including the coil 5and bat tery 16, is now connected directly with the tube 18.

The operation As has been stated, the use of the invention is describedas in connection with a talking circuit. It will obviously operate withequal facility when used in a circuit which is subject to vibrationsother than those of the voice. For example, the telephone transmitter 7may be replaced by a telegraph key. The principle of the modulatorremains the same regardless of what the origin of the means thatprodudes the changes of current value in the primary portion of thetalking circuit 6.

Assume first that the diaphragm of the telephone 7 is idle. Theoscillator 2 is regarded as being in operation and the amplitude of thecurrent impressed upon the an tenna 1 is therefore constant. Both therheostats 10 and 17 are adjusted to a given set-- ting. The currentvalue in the primary talking circuit will therefore be constant so thatthe heat given off by the filament 9 will be constant. Similarly, thecurrent value in the secondary talking circuit (including the pri marycoil 5 of the transformer) will be constant.

Assume that the telephone transmitter 7 is spoken into. There is animmediate change in both the primary and secondary portions of thetalking circuit 6 resulting in the production of an E. M. F. across theterminals of the secondary transformer coil 12. The grid of theoscillator 2 will therefore be impressed not only with the highfrequency current due to the interaction of the coils 19 and 20 of theoscillator circuit but also the low frequency E. M. F. in the talkingcircuit due to the speech.

It is plain that the low-frequency E. M. F. of the talking circuit willresult in an increase or decrease of the grid potential above or belownormal, and inasmuch as the grid potential reacts upon the antennacircuit by means of the oscillator 2 itself and the coils 21 and 19, itis evident that the amplitude of the antenna current will be changed inaccordance with the varying M. F. of the coil 4, or getting back to theorigin, of the talking circuit 6. The foregoing constant values of theprimary and secondary portions of the talking circuit undergo a changewhen the telephone diaphragm is vibrated. The immediate result of suchvibrations is an increase or decrease of current flow from the battery 8and an increase or decrease of heat at the filament 9. The next resultis an increase or decrease of conductivity of the substance 13 so thatthe flow of current from the battery 16 in the secondary talking circuitis either facilitated or restricted. The next result is the productionof a variable E. M. F. across the terminals of the secondary transformercoil 4 with the effect previously described.

Similarity of principle prevails in Figure 3. The substance of the tube18 is affected by the heat of the filament 9, and as the heat varies,the conductivity of the tube 18 varies in turn so that a regulation ofthe current from the battery 16 is accomplished in the manner clearlyunderstood.

l'Vhile the construction and arrangement of the improved modulator isthat of a generally preferred form, obviously modifications and changesmay be made without departing from the spirit of the invention or thescope of the claims.

I claim 1. In a system of radio communication, an oscillator including agrid circuit, said oscillator having an associated antenna, and amodulator for varying the amplitude of the oscillator current impressedupon the antenna, comprising a transformer having one side connected inthe grid circuit of said oscillator, a microphone circuit including afilament, and means connected with the other side of the transformerbeing variable in electrical conductivity by variations in the lightemitted by the filament.

2. In a system of radio communication, an oscillator including a gridcircuit, said 0scillator having an associated antenna, and a modulatorfor varying the amplitude of the oscillator current impressed upon theantenna comprising a transformer having one side connected in the gridcircuit of said 0scillator, an electrically energized primary microphonecircuit including a filament, an electrically energized secondarycircuit connected with the other side of the transformer, and a resistorincluded in the secondary circuit being associated with the filament andvariable in its conductivity by current fluctuations in the filament.

HENRY H. C. DUNVVOODY.

